The invention relates to a flat gasket including at least one inner eyelet or at least one outer eyelet or at least one inner and one outer eyelet, as well as a gasket body. In each case the inner eyelet is positioned at a transition to each cavity to be sealed by the gasket. The gasket body having the at least one eyelet is formed of a pressure-resistant core which has long-term resistance to temperatures of at least 150xc2x0 C. and a covering that is present on both flat outer sides of the core.
Flat gaskets are used in industry in order to seal spaces, such as pipelines, containers, reaction spaces, etc. containing fluid products, at locations at which such spaces have at least one transition to another structural or functional part of a technical assembly to which they belong and which is materially separated from the spaces. It is their function, where there are pressure differences between an interior of the container and a space outside the container to prevent an escape of fluids from such a container or system of containers or an unwanted penetration of fluids into the container or system of containers, as far as possible.
Moreover, even in the absence of pressure differences, exchanges of materials between the interior of the container and the exterior of the container as a result of diffusion should be prevented as far as possible. For economic reasons, for reasons of public health and because of the necessity for environmental protection, the demands placed on the efficiency and especially on the impermeability of flat gaskets have steadily increased in recent times and that process continues. If leakage limiting values in the vicinity of 0.01 mg/(m s) were adequate in the past, nowadays values of 0.0001 mg/(m s) are under discussion for the field of mineral oil processing. As a rule, such high requirements for impermeability can no longer be met by conventional single-material or single-layered flat gaskets. Sealing systems built up from several layers of different materials are also known. U.S. Pat. No. 5,128,209 describes a gasket material formed of layers of a fluoropolymer, graphite foils and metal foils, wherein the layers are bonded to one another by an adhesive. The object of developing that gasket material was to make a gasket which would be easy to handle and have a high spring back capacity. The criterion of impermeability was of secondary importance in that case. The layers of the fluoropolymer are formed of material which is porous and therefore permeable to fluids. They impart a greater stability, toughness and tensile strength and an improved ease of handling to that component of the composite formed of the graphite foils and the fluoropolymer foils. A great disadvantage of that gasket material is the permeability of the fluoropolymer foils to fluids combined with the presence of adhesives as bonding agents between the layers. The fluoropolymer foils contribute virtually nothing to the impermeability of the overall system, which has an adverse effect, especially in the case of gaskets for high and very high impermeability requirements. The adhesives can be a weak point, especially in the case of gaskets subjected to high pressures or stresses. That is because the layers of the laminate can slide on the adhesives, which can ultimately lead to the non-functioning of the gasket or, particularly in combination with more elevated temperatures, the adhesive layer can develop fine cracks which impair the sealing action. German Utility Model G 92 08 943.7 discloses packing rings for packed glands. The rings are formed of layers of graphite foils and of metal foils and have an at least partial covering of a foil of polytetrafluoroethylene as a diffusion barrier. The polytetrafluoroethylene foils can also be sintered onto the graphite rings. However, the mechanisms which bring about the sealing in gland packings are not readily transferable to flat gaskets. A gland packing is completely enclosed in its packing seat and is pressed against walls of the packing space surrounding it by tightening the packing seat. Unlike the case of flat gaskets, flow processes of the packing ring are provided for in that case and can only go as far as the space surrounding it allows. The metal rings disposed in the packing serve primarily as diffusion barriers between the graphite layers. The graphite layers have free diffusion paths in all directions. In flat gaskets the conditions are different. They are not completely enclosed and can have no, or only a very limited, tendency to flow where there is stressing of the sealing surfaces such as, for example, flanges, forming a boundary to them above and below. Fine channels and diffusion paths which pass horizontally through such gaskets control the leakage rate of those gaskets. Such diffusion paths are virtually absent in gaskets which have a core constructed only of plastics materials or of metal. However, gaskets of that type cannot meet the technical sealing demands placed on them, or only inadequately do so. In gaskets containing a pure plastics core, the plastics core flows under pressure and lacks an adequate spring back capacity. In gaskets containing a pure metal core, the metal core does not flow but it has no spring back capacity at all from the aspect of sealing technology and consequently does not have a reliable long-term sealing action. Other materials have therefore been used as gasket cores in order to surmount those difficulties. For example, such materials may be plastics having a certain elasticity, or special rubbers, for instance nitrile-butadiene rubber, which on one hand have been reinforced with fillers and by incorporating reinforcing fibers such as aramid fibers or carbon fibers, and on the other hand have been made flow-resistant or creep-resistant, or else inorganic materials such as, for example, graphite foils or graphite laminates. However, gasket cores of that type also have diffusion channels, as has been demonstrated, for instance, in Published European Patent Application 0 676 570 A1 by the example of graphite foil gaskets. In summary, it can be stated that the known flat gaskets do not completely meet the requirements of modern sealing technology with regard to impermeability and compressive strength and are in need of improvement.
It is accordingly an object of the invention to provide a gasket with an eyelet, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which provides a flat gasket having high compressive strength, that is suitable for long-term temperature stability of at least 150xc2x0 C. and through the use of which improved leakage values, that is of less than 0.01 mg/(m.s), are achieved. With the foregoing and other objects in view there is provided, in accordance with the invention, a flat gasket for sealing at least one cavity, comprising a gasket body including a pressure-resistant core having long-term resistance to temperatures of at least 150xc2x0 C. and two flat outer sides, and coverings each completely covering a respective one of the flat outer sides, the coverings made of a gas-tight foil of an organic polymer having a long-term temperature resistance of at least 150xc2x0 C.; and at least an inner eyelet or at least an outer eyelet or at least one inner and one outer eyelet or eyelet sheet, positioned at a transition between the gasket body and the at least one cavity and enclosing the gas-tight foil of the gasket body with a gas-tight joining.
Besides their low permeability to fluids, the polymer foils covering the core of the gasket body have yet another important property. Due to their good flow performance under pressure, they adapt themselves extremely well to the unevennesses and damage which are present on the surfaces to which they are sealed and thus also effect an excellent sealing against them. If they are used in a sufficiently low thickness in this case, their tendency to creep, which per se is disadvantageous for gaskets, and their low spring back capacity, are of no consequence, because the adverse effects caused thereby are more than compensated for by the core which, as will be shown below, has a good spring back capacity.
The polymer foils forming the boundary at the two flat surfaces of the multilayer sheet and completely covering it must have a long-term temperature stability of at least 150xc2x0 C. They preferably have a long-term temperature stability of at least 200xc2x0 C. and particularly preferably a long-term temperature stability of 250xc2x0 C. Within the context of this invention, long-term temperature stability means that, at the specified temperature, the respective foil neither melts nor noticeably decomposes in air. All foils which possess these features are suitable for use as a component of the multilayer sheet according to the invention.
Nevertheless, in accordance with another feature of the invention, the foils are preferably formed of a material selected from the group including polyarylether ether ketone, polyaryl ether ketone, polyphenylene sulfide, polyaryl sulfone, polyether sulfone, polyimide, polyamide imide and fluorine-containing polymers. Particularly preferred materials are fluorine-containing polymers from the group including polytetrafluoroethylene, polytrifluorochloroethylene, tetrafluoroethylenehexafluoropropylene copolymer, copolymers of tetrafluoroethylene with perfluoroalkyl vinyl ether, copolymers of ethylene and tetrafluoroethylene and polyvinylidene fluoride. Of these foils, those formed of perfluorinated organic polymers are again preferred.
In accordance with a further feature of the invention, the polymer foils contained in the multilayer sheet have a thickness within a range of 0.005 to 1.0 mm, with thicknesses within a range of 0.03 to 0.2 mm being preferred.
In accordance with an added feature of the invention, in a first variant of the invention, the core of the gasket body can be formed of a thermoplastic synthetic material or of an elastomer reinforced by organic or inorganic fibers, preferably fibers having a length of not more than 20 mm. If the compressive strength of the plastics or elastomer body which is thus reinforced is inadequate, it can be brought to the required value by an additional content of a filler such as, for example, silicon dioxide, aluminum oxide, carbon, graphite in powder form or stone dust, ceramic dust or wood dust.
In accordance with an additional feature of the invention, the reinforcing fibers of the core are preferably aramide fiber, carbon fiber, graphite fiber, glass fiber, ceramic fiber or metal fiber.
In accordance with yet another feature of the invention, the thermoplastic synthetic material is a plastics material selected from the group including polyarylether ether ketone, polyaryl ether ketone, polyphenylene sulfide, polyaryl sulfone, polyether sulfone, polyimide, polyamide imide and fluorine-containing polymers. In particular, it is a plastics material selected from the group including polytetrafluoroethylene, polytrifluorochloro-ethylene, tetrafluoroethylene-hexafluoropropylene copolymer, copolymers of tetrafluoro-ethylene with perfluoroalkyl vinyl ether, copolymers of ethylene and tetrafluoroethylene and polyvinylidene fluoride. The elastomer used is preferably a material selected from the group including nitrile-butadiene rubber, styrene-butadiene rubber, natural rubber, ethylenepropylene terpolymer, ethylene-vinyl acetate copolymer, fluorinated rubber, fluorosilicone rubber, vinyl-containing dimethylpolysiloxane, acrylic rubber and ethylene-acrylic rubber.
In accordance with yet a further feature of the invention, in a second and preferred variant of the invention, the core of the gasket body is formed of an inorganic and pressure-resistant material selected from the group including graphite and mica. In a particularly preferred variant, it is formed of a variety of graphite which has been produced from expanded graphite, is available in foils or in laminate form and is well-known to the person skilled in the art, or it contains such a variety of graphite. For the purpose of this invention, the term xe2x80x9cgraphite foilsxe2x80x9d means both graphite foils and graphite laminates. These are obtained by known processes by compression or calendering of expanded, so-called vermicular graphite. Expanded graphite is produced by sudden decomposition of graphite salts, such as graphite hydrogen sulfate, at elevated temperatures. Vermicular graphite can be produced, for example, by the process disclosed in U.S. Pat. No. 4,091,083.
In accordance with yet an added feature of the invention, the graphite foils used for the production of gasket bodies according to the invention and situated in these gasket bodies can have a bulk density within a range of 0.1 g/cm3 to 1.8 g/cm3. The graphite foils situated in the gasket bodies more probably have bulk densities which are in the middle and upper range of the specified bulk density values. They have a carbon content preferably within the range of 90 to 99.95 per cent by weight. Their thickness is within the range of 0.1 mm to 4 mm. Where graphite foils alone are used, the core of the gasket body may be formed of only one layer of graphite foil or of several layers disposed one above the other.
In accordance with yet an additional feature of the invention, the core is formed of different layers disposed alternately on top of one another and in parallel, including a layer made of an inorganic, pressure-resistant material which is resistant to high temperatures and metal foils. It is advantageous if the metal foils in the core are joined to the layers of inorganic, pressure-resistant material which is resistant to high temperatures. This bond can be produced by an adhesive which is resistant to temperatures of above 150xc2x0 C. In a more developed variant of this structure of the core, the two layers directed to the outside, that is the upper and the lower layer of the core, are each formed of a metal foil completely covering the flat surfaces facing towards it of the layers of the core adjacent it.
In accordance with again another feature of the invention, the inorganic, pressure-resistant material which is resistant to high temperatures is formed of mica. In a particularly preferred embodiment of this variant of the invention, the inorganic, pressure-resistant material which is resistant to high temperatures is formed of at least one graphite foil. In order to provide particularly efficient gaskets, there is a joining of the graphite foils to the metal foils and there is also a joining of the metal foils to the polymer foils of the covering, which are free of adhesives.
In accordance with again a further feature of the invention, in all embodiments containing graphite foils and metal foils, the metal foils which are not joined to polymer foils of the covering can be shaped in the form of tanged metal sheets, from which tines extend either on only one side or on both sides. Preferably, only the metal foil forming the center of the core is a tanged metal sheet having tines on both sides, which are anchored into the two adjacent graphite foils.
In one method for joining metal foils and graphite foils without an adhesive, the two foils are brought into contact at the surfaces by which they are to be bonded together and are compressed by applying pressure and temperatures within the range of 150xc2x0 C. to 300xc2x0 C. In the case of smaller expanses of surfaces, this can be done in stamping presses. In the case of large lengths, heatable twin-belt presses or roll mills are used. If one wishes to produce an extremely firmly adhering joining between the metal foils and the graphite foils, one will use the process according to European Patent EP 0 616 884 B1, corresponding to U.S. Pat. No. 5,509,993, as another method which is preferred. In that method, the surfaces to be joined together are coated with as thin a layer as possible of a substance acting per se as a separating agent and the joining is produced by pressure and the effect of temperature.
In accordance with again an added feature of the invention, a flat gasket regarded as advantageous because of its limited number of layers, has a gasket body formed of a centrally disposed metal foil, two layers of graphite foil joined to the flat surfaces of this metal foil and two metal foils joined to the flat outwardly-directed surfaces of the two graphite foils, wherein the flat outwardly-directed surfaces of these metal foils are completely joined to a polymer foil. Another gasket of this type has a gasket body made of a centrally disposed graphite foil, both of the flat surfaces of which are each joined to a metal foil, and the two outwardly-directed surfaces of which are completely covered with a polymer foil.
The core of the gasket body is always joined to the covering. In order to produce this joining, either an adhesive is used or a joining is produced without an adhesive. Adhesives are used in particular for bonding polyimide foils to the core. The adhesives that are used can be commercially available adhesives which are resistant to temperatures of above 150xc2x0 C., such as silicone adhesives or acrylic adhesives. Joinings without an adhesive are preferred, since the layers of adhesive invariably constitute a certain weakness in the gaskets. That is because in those layers they can form fine cracks which act as diffusion channels, or because adhesives which do not cure to form thermosetting networks, under high pressure and possibly at elevated temperatures, act as a lubricant for the layers of material which are bonded to one another. Such a joining without an adhesive between the core and the covering is formed by pressing the polymer foil forming the covering to at least one layer of the core or to a foil which will subsequently become a component of the core, such as, for example, a metal foil or a graphite foil, at a temperature at which the polymer foil softens and at a pressure at which the joining can then be produced. In the course thereof the core becomes welded to the polymer foil.
This process can also be used for foils made of polytetrafluoroethylene. Stamping presses as well as twin-belt presses or roll presses are suitable for the production of joinings of this kind. In order to provide economical operation, it is advantageous to use pressing devices in which at least one of the foils to be joined, for example by tempered rolls, can be heated and in which the pressure required for joining can be applied continuously.
In accordance with again an additional feature of the invention, the metal foils of the gasket body may be formed of any metal which is capable of being used for sealing purposes and of being produced in foil form. It is, however, preferable to use metal foils made of aluminum, aluminum alloys, copper and copper alloys and of a corrosion-resistant metal or of a corrosion-resistant metal alloy such as, for example, foils made of special steel. The metal foils have a thickness preferably within a range of 0.005 to 1 mm. In special cases the thickness may be outside this range.
The advantageous properties of the gasket body are most effective in the flat gasket only if at least the inner eyelet is, or the inner eyelets are, constructed and attached in such a way that it or they surround the gasket body completely and render it gas-tight. The eyelet or eyelets can be attached through the use of known devices. The eyelets can be formed of metal or of a suitable plastics material. Preferably, eyelets made of a corrosion-resistant metal or of a corrosion-resistant metal alloy are used. Eyelets made of plastics material are preferably formed of a perfluorinated organic polymer. In this case a conclusive joining, produced by welding or contact adhesion, between the eyelet and the polymer foil forming the covering, is advantageous for producing gas-tight conditions.
In accordance with a concomitant feature of the invention, the flat gaskets have at least a leakage rate in accordance with DIN 3535 of less than/equal to 0.1 ml/min.
Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a multilayered gasket with an eyelet, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.